Upgrade Late 2013 iMac to 16GB or 24GB?

So I've been going ok with the stock 8GB for about a year now but now that I'm using Parallels more I'm finding that it is now a tight squeeze. I am considering upgrading to 16GB or 24GB. I like the idea of just adding a 2x4GB kit to make all the banks full with the same memory dimensions for a total of 16GB but wondering if I might as well just go with 2x8GB kit for a total of 24GB.

I plan on keeping this computer for about another 2 years as I usually upgrade every 3 years.

Also, there seems to be a difference in part numbers between crucial's web site and amazon's. Amazon is cheaper but I feel like it might be better to just go direct with Crucial? Which would you go with?

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